In an ultrasonic object detection device, an ultrasonic sensor transmits an ultrasonic wave, and receives a reflected wave stemming from reflection of the ultrasonic wave from an object. The ultrasonic wave to be transmitted shall be referred to as a transmission wave. A receiving wave obtained immediately after the transmission wave is transmitted exhibits higher signal strength because of an echo of the transmission wave.
Patent literature 1 discloses that when a reflected wave is present because an object or the like exists at a near distance, the wave reflected is added to an echo wave caused by transmission and the duration of the echo wave gets prolonged.
In the patent literature 1, designation of a reference time precedes determination on presence or absence of an object. As the reference time, a termination time of a receiving wave signal to be received concurrently with transmission of an ultrasonic wave is designated. The designated reference time is equivalent to an echo time elapsing when an object is absent.
As for determination on presence or absence of an object, when the termination time of a receiving wave signal to be received concurrently with transmission of an ultrasonic wave gets longer than the reference time, a near-distance object is determined to be present.
In the patent literature 1, when the echo time measured when whether an object is present or absent is determined gets longer than the reference time designated prior to determination on presence or absence of an object, a near-distance object is determined to be present. However, even when an object exists at a near distance, the echo time may not get longer. Therefore, the technology in the patent literature 1 cannot precisely detect an object existing at a near distance.